Chemically active glasses for steel enamels

ABSTRACT

A corrosion resistant steel reinforcing rod system, including a steel reinforcing rod having a coefficient of thermal expansion of between about 14 ppm/° C. and about 17 ppm/° C. and a vitreous shell substantially encapsulating the steel reinforcing rod. The vitreous shell has a composition selected from the group consisting essentially, in weight percent, of about 40-45% SiO 2 , 3-5% Al 2 O 3 , 5-15% B 2 O 3 , 3-15% K 2 O, 5-20% Na 2 O, 4-7% CaO, 1-2% ZrO 2 , 0-2% NiO, 0-2% CoO, and 5-20% P 2 O 5 . The vitreous shell has a coefficient of thermal expansion between about 12.5 ppm/° C. and about 13.5 ppm/° C.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority to co-pending U.S. patentapplication Ser. Nos. 13/040,781, filed Mar. 4, 2011, which claimspriority to Ser. No. 12/623,236, filed Nov. 20, 2009, and issued on Mar.8, 2011 as U.S. Pat. No. 7,901,769, which claims priority to U.S.Provisional Patent Application Ser. No. 61/199,901, filed Nov. 21, 2008.

GRANT STATEMENT

The invention was made in part from government support under Grant No.W911NF-07-2-0062 from the Department of the Army. The U.S. Governmenthas certain rights in the invention.

TECHNICAL FIELD

The present invention relates to structural materials and, moreparticularly, to a new and improved glass composite developed forcoating steel elements for reinforcing concrete structures.

BACKGROUND

One material very commonly selected for large-scale constructionprojects is reinforced concrete (RC). Several years ago, it wasdiscovered that the use of a modified vitreous enamel improved the bondstrength, and, possibly, the corrosion resistance of the steel rodsreinforcing the concrete. The enamel consisted of a glass matrixembedded with reactive ceramic particles. The glass composition wasfound to strongly adhere to the steel, and the reactive particles wereimbedded to chemically react with the surrounding cement to form anotherstrong bond.

The materials used for these initial tests included commercialalkali-resistant groundcoat enamelss for steels used in a variety ofconsumer and industrial applications. The typical compositional rangesfor such enamels are summarized below as Table 1.

TABLE 1 Compositional ranges for typical alkali-resistant groundcoatsConstituent Range (wt %) Silicon dioxide SiO₂ 40-45 Boron oxide B₂O₃16-20 Na oxide Na₂O 15-18 K oxide K₂O 2-4 Li oxide Li₂O 1-2 Ca oxide CaO3-5 Aluminum oxide Al₂O₃ 3-5 Zr oxide ZrO₂ 4-6 Mn dioxide MnO₂ 1-2 Nioxide NiO 1-2 Cobalt oxide Co₃O₄ 0.5-1.5 Phosphorus oxide P₂O₅ 0.5-1 

The ratio of the Na₂O, B₂O₃, and SiO₂ components, as well as theaddition of other alkali (K₂O and Li₂O) and alkaline earth oxides (CaO),have the greatest effect on the thermal properties of the glass.Constituents like Al₂O₃ are added to improve the corrosion-resistance ofthe glass. ZrO₂ is usually added to an enamel as an opacifier to affectthe visual appearance of the coating. However, zirconia has the addedadvantage of improving the chemical resistance of silicate glasses toattack by alkaline environments. Alkaline-resistant silicate glassfibers developed for reinforcing cement composites typically contain10-20 wt % ZrO₂, and a protective coating of Zr-oxyhydroxide forms onthe glass surface when exposed to an alkaline environment, furtherimpeding corrosion. Transition metal oxides, like MnO₂, Co₃O₄, and NiO,are added to enamels to aid bonding to the substrate.

In general, these materials are sodium-borosilicate glasses modifiedwith various constituents to tailor thermal and chemical properties.However, the conventional groundcoat enamels (such as the ones listed inTable 1) only offer limited protection to the underlying steel if theyare cracked or otherwise damaged. If the cementitious material candirectly contact the steel, such as through chips, holes or cracks inthe glass or enamel coating, the steel is then locally attacked by thecorrosive cementitious material. Therefore, there is a need to provide anew and improved glass or enamel composition offering enhanced corrosionresistance to the steel, as well as corrosion resistance even if crackedor broken. The present novel technology addresses these needs.

SUMMARY

The present novel technology relates to a chemically-active glasscomposition for providing corrosion protection to coated steels for usein alkaline environments.

One object of the present novel technology is to provide an improvedsteel reinforced concrete system including the same. Related objects andadvantages of the present novel technology will be apparent from thefollowing description.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cutaway perspective view of a steel rod coated with avitreous material according to a first embodiment of the present noveltechnology.

FIG. 2A is a perspective view of a first plurality of steel rodsaccording to FIG. 1 embedded in a cementitious material to yield a firstcomposite material according to a second embodiment of the present noveltechnology.

FIG. 2B is an enlarged partial view of one of the embedded rods of FIG.2A.

FIG. 3A is a perspective view of a second plurality of steel rodsaccording to FIG. 1 embedded in a cementitious material to yield asecond composite material according to a second embodiment of thepresent novel technology.

FIG. 3B is an enlarged partial view of one of the embedded rods of FIG.2A.

FIG. 4 shows weight changes for glasses after up to 28 days in alkalineLawrence Solution at 80° C.

FIG. 5 shows the comparisons of average bond strengths (in MPa) forsteel pins embedded in mortar after up to 60 days.

FIG. 6 is a graphical representation of the change in linear dimensionvs. temperature of a steel rod and two vitreous coating compositions forthe coated steel rods of FIG. 1.

FIG. 7 is a cutaway perspective view of a steel rod coated with aphosphate-releasing glass according to a third embodiment of the presentnovel technology.

FIG. 8A is a perspective view of a second plurality of steel rodsaccording to FIG. 7 embedded in a cementitious material.

FIG. 8B is a first enlarged is an enlarged partial view of one of theembedded rods of FIG. 8A illustrating direct contact of an exposedportion of a steel rod with cement.

FIG. 8C is a second is an enlarged partial view of one of the embeddedrods of FIG. 8A illustrating the formation of a barrier patch at theexposure site.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of thenovel technology and presenting its currently understood best mode ofoperation, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of thenovel technology is thereby intended, with such alterations and furthermodifications in the illustrated device and such further applications ofthe principles of the novel technology as illustrated therein beingcontemplated as would normally occur to one skilled in the art to whichthe novel technology relates.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs.

In one embodiment, steel reinforcing rods 10 are coated with the novelglass composition 20 to yield coated reinforcing rods 30. The glasscoating 20 is particularly suitable for coating the steel alloys used inthe rods 10, as the glass coating 20 typically has a coefficient ofthermal expansion close to but lower than that of the steel rods 10,such that the glass coating 20 is maintained in compression. Further,the glass coating 20 is substantially more corrosion resistant than theconventional enamel coatings known in the art. Specifically, the thermalproperties of the glass coatings are tailored for the steel alloys usedin RC structures, which have different thermal expansion coefficientsthan the alloys used in commercial and industrial applications for whichthe conventional groundcoat compositions were designed. Typically, thesteel alloys used in the rods 10 are ASTM A 615, 706, 955, 996 or thelike, which typically have thermal expansion coefficients of from about14 ppm/° C. to about 17 ppm/° C. The glass coating 20 typically has athermal expansion coefficient of between about 12.5 ppm/° C. and about13.5 ppm/° C. at ambient temperatures typical of most applications.

In particular, the borate-to-silicate ratio and the fraction and type ofalkali oxide of the coatings 20 has been optimized to yield coatings 20characterized by greater CTE to improve the thermomechanicalcompatibility with typical reinforcing steel. In other words, the CTE ofthe glass coatings 20 has been raised to be closer to that of typicalsteel rebars 10 while remaining slightly lower than the steel CTE, suchthat the glass coating 20 is put into compression 20 but not so much sothat it fails and disengages therefrom. Further, this CTE matching wasaccomplished without sacrificing chemical durability of the glasscoating 20. Thus, by better matching the thermomechanical properties ofthe glass coatings 20 to the steel members 10, the glass coatings 20 areless prone to failure due to stresses arising from thermal cycling andthus remain on the steel members 10 where they can participate in thebonding process with a surrounding cementitious matrix material.

Additionally, the corrosion resistance of the glass coatings 20 isespecially attractive in alkaline environments. The glass coatings 20typically includes substantially increased concentrations of CaO, K₂Oand, more typically, ZrO₂ at levels substantially greater than thetypical enamel compositional ranges to provide increased corrosionresistance of the glass coated rods 30 in alkaline environments.

In some embodiments, as seen in FIGS. 2A-3B, cement-reactive particles35, such as calcium silicate, are dispersed in the glass coatings 20 toenhance bonding with a cement matrix 40 to result in a steel-reinforcedconcrete composite material 50 having increased bond strength betweenthe coated rods 30 and the cement matrix 40. Such a material 50 willexhibit a substantially increased pull-out strength and be inherentlytougher. Optionally, metal particles 45 such as zinc may be dispersed inthe glass coating 20 to act as sacrificial anodes for further protectingthe steel rods 30 from the corrosive effects of the cementitious matrix40. Still optionally, such sacrificial anode particles 45 may be addeddirectly to the cement, either throughout or preferentially near thesteel rods 10, to react locally with the corrosive cementitious matrix40 to divert its attack on the steel rods 10. As they are corroded, thesacrificial metal particles 45 will expand to provide both physical aswell as chemical protection, chemically reacting with corrosives andphysically blocking the corrosion pathways.

Table 2 shows the compositions of several embodiments of the glasscoating 20, along with test results of the dilatometric softening pointand the CTE, designated ARE-1 through ARE-5. For comparison, thecomposition and properties of a standard (conventional) alkali-resistantgroundcoat composition is presented and designated ARG.

TABLE 2 Comparision between the novel glass coating compositions and ARGARE- ARE- ARE- ARE- ARE- ARE- wt % 1 2 3 4 5 11 ARG SiO₂ 44.5 43.4 39.742.0 33.2 39.3 44.0 B₂O₃ 17.9 14.4 14.0 13.9 19.2 13.0 19.3 Na₂O 15.915.5 15.1 8.9 8.6 8.3 15.8 K₂O 4.3 4.2 4.1 13.5 13.0 12.6 2.8 CaO 5.15.0 4.8 4.8 4.6 4.5 4.7 CaF₂ Al₂O₃ 3.6 3.8 3.7 3.6 3.5 3.4 4.6 ZrO₂ 5.610.9 10.6 10.6 15.3 9.9 5.3 MnO₂ 0.7 0.6 0.6 0.6 0.6 0.6 1.5 NiO 1.1 1.11.1 1.1 1.0 1.0 1.0 CoO 1.1 1.1 1.1 1.1 1.0 1.0 0.9 P₂O₅ 0 0 0 0 0 6.4Soft Temp 600 586 600 600 594 610 576 (° C.) CTE 13.5 12.9 12.5 12.912.7 10.8 12.2 (ppm/° C.)

FIG. 4 shows the change in weight for glass samples after up to 28 daysat 80° C. in Lawrence solution (pH=13). The K₂O and ZrO₂ contents of theARE-series glass coatings 20 are each, respecitvely, greater than thoseof the ARG composition, and the weight changes of ARE compositions 2 and5 are respectively less than that of the ARG glass.

In another embodiment, reinforced concrete 50 was prepared by thepouring wet concrete over coated rods 30 and allowing the concrete todry and cure to define a concrete matrix 40, yielding a reinforcedconcrete composite material 50. The bonding of the coated rods 30 in theconcrete matrix 40 was analytically measured.

A series of pull-out tests was conducted to assess the bond strengths ofthe embedded coated rods 30 with several compositional embodiments ofthe glass coating material 20. The results of pull-out testing are shownin FIG. 5.

Preparation of Test Mortar.

Uncoated steel rods 10 and coated rods 30 were embedded in a mortarprepared using the guidelines presented in ASTM C109, Standard Methodfor Determining Compressive Strength of Hydraulic Mortars. Theproportion of the standard mortar was one part cement (Type I) to 2.75parts of standard graded sand. The water-to-cement ratio was maintainedat 0.485. Test cylinders were prepared for each mortar batch and testedto investigate the compressive strength at 7 and 30 days.

Preparation and Testing of Embedded Rods for Pull-out Testing.

Each uncoated 10 and glass coated test rod 30 was inserted in a 50.8-mmin diameter, 101.6-mm long plastic cylinder mold filled with freshmortar. The respective rods 10, 30 were clamped at the top so that a63.5-mm length of each respective rod 10, 30 was under the mortar; forthe coated rods 30, the portion under mortar was glass coated. Eachcylinder was tapped and vibrated to remove entrapped air and also toconsolidate the mortar. The samples were kept in a 100% humidityenvironment at room temperature and cured, with curing times rangingfrom 7 days and to 60 days. After curing, the test cylinders werede-molded and the mounted in the test apparatus and the force requiredto pull each respective rod 10, 30 out of the mortar was measured.

The testing pin-pull results for steel after up to 60 days in mortarindicate that the bond strength of the uncoated pins decreases fromabout 4 MPa to about 2.2 MPa between seven days and 28 days of curing.This is consistent with reports in the literature for decreasing bondstrength between cement paste and reinforcing steel with increasingcuring time age, particularly from 1 to 14 days. However, due thehydration reaction of cement with the reactive Ca-silicate particlesused for the glass coated samples 30, these bond strengths increase from1.2 MPa to 6.60 kPa with an increase of curing time from three days to60 days. Further, glass coated steel pins 30 with reactive calciumsilicate have about three times the bond strength of bare steel pinsafter 60 days in cement.

Steel-reinforced concrete composite material 50 benefitting fromincreased bond strength and decreased degradation of the steel 10 fromcorrosive attack by the concrete matrix 40 give rise to a number ofuses, such a structural material for floors and decking, hardened orreinforced civilian and military structures, sewage pipe, geotechnicalanchorages, and the like. Further, the strong bond formed between theglass-coated steel 30 (with or without calcium silicate particles or thelike dispersed therein as bonding enhancers) and the cementitiousmaterial 40 enables design options such as concrete-filled steel tubesor casings.

Further, the glass composition may be optimized to be self-sealing. Asthe glasses have relatively low softening temperatures, they are wellsuited for low temperature applications, such as retrofit andremediation applications. Additionally, glass-tape composites may bemade with these compositions that may be wrapped around steel membersand then fused thereto via the direct application of heat, such as byinduction or a torch, to provide corrosion protection and/or an enhancedbonding surface.

Referring to FIG. 6, the change in linear dimension as a function oftime is plotted for both a steel rod 10 and for two coating compositions(ARE-4 and ARE-11P). The rod 10 has a measured CTE of 16.9 ppm/degreesCelsius, while the ARE-4 composition has a CTE of 12.9 ppm/degreeCelsius and the ARE-11P has a CTE of 10.8 ppm/degree Celsius. The CTE ofthe rod 10 is substantially constant over a temperature range of about100 to about 700 degrees Celsius, while the CTE's of the glass coatingcompositions are substantially constant over ranges of between about 200to about 450 degrees Celsius. Both compositions appear to beging tosoften at about 500 degrees Celsius, resulting in a change in CTE in the500 to 600 degree Celsius range.

The desired properties of the novel glass composite include 1) acoefficient of thermal expansion (CTE) that is more compatible with thesteel alloy that is to be coated, 2) a softening temperature that isrelatively low (<700 degrees Celsius) to ensure low processingtemperatures that do not degrade the mechanical properties of the steel,and 3) outstanding corrosion-resistance to the alkaline environment ofwet cement. The novel glass composite comprises at least 4.0% (wt) K₂Oand at least 5.6% (wt) ZrO₂, with about 4-20% (wt) of K₂O, and/or about5-20% (wt) of ZrO₂, whereas both K₂O and ZrO₂ are significantlyincreased compared to the conventional groundcoats.

In another embodiment, as shown in FIGS. 7-8B, chemically-active glasscompositions 100 may be used to coat steel rods 10 to provide enhancedcorrosion-protection steel members 130 used for reinforcing cementitiousmatrices 40 or reinforced concrete 50, while still providing the desiredcharacteristics (e.g., thermal properties like coefficient of thermalexpansion and softening temperature) for use with the composite enamelsto enhance steel-concrete bond strengths. Specifically, the glass 100releases phosphate anions when exposed to a corrosive alkalineenvironment, such as that characteristic of a cementitious matrix 40.The phosphate anions react at exposed surfaces of steel 10 to form abarrier against corrosion.

The chemically-active glass compositions 100 provide added protectionfor steel rods 10 used in RC structures. The glasses 100 may be used asthe frit component in a reactive enamel formulation for enhanced bondstrengths between the concrete 40 and steel 10. The glasses 100 containrelatively high concentrations of P₂O₅ (typically between about 5 andabout 20 weight percent, more typically between about 9 and about 18weight percent), far in excess of the concentrations than used in theconventional groundcoat compositions (typically about 0.5-1 weightpercent, see Table 1). In the present novel technology, P₂O₅ is releasedas phosphate ions when attacked by a corrosive aqueous alkalinesolution. The released phosphate anions react with calcium cations, andother constituents in the corrosive environment, to form a stablehydroxyapatite (HAp) coating 110 on exposed metal, such as steelreinforcing rods 10. This HAp coating 110 provides protection of themetal 10 against corrosion. Representative compositions ofphosphate-releasing glasses 100 are given in Table 2. The glasses 100are designated as ‘ARE-xP’ glasses, where ‘x’ is a composition number,and ‘P’ signifies a phosphate-releasing composition. The coefficient ofthermal expansion (CTE) and dilatometric softening temperature for eachglass is also listed. These properties were determined by dilatometry.

TABLE 3 Example compositions and selected properties ofphosphate-releasing glasses for reactive enamel coatings of steel in RCstructures wt % ARE-7P ARE-8P ARE-9P SiO₂ 41.5 41.9 41.7 P₂O₅ 17.1 13.49.1 B₂O3 8.3 10.4 13.8 Na₂O 14.9 15.0 9.2 K₂O 4.0 6.7 14.1 CaO 6.3 4.84.8 Al₂O₃ 3.6 3.6 3.6 ZrO₂ 1.7 1.8 1.7 MnO₂ 0.0 0.0 0.0 MO 1.6 1.6 1.1CoO 1.1 1.1 1.1 Soft. Temp (C.) 625 621 CTE (ppm/C.) 16.5 13.1 logWtchange (g/cm2-min)

Microscopic and spectroscopic analyses of the surfaces of the steel 10enameled with phosphate-releasing glasses, such as the ARE-8Pcomposition, as well as phosphate-free glasses, such as the ARE-4composition, show that the phosphate-releasing glass 100 providesenhanced corrosion protection to the steel 10, even in regions whereglass 100 did not initially coat the steel 10. For example, micro-Ramanspectroscopy revealed that holes 125 in the ARE-8P enamel coating 100 ofup to several hundred microns were filled with HAp after 24 hours inLawrence solution, a simulated cement effluent, whereas only rust (ironoxide) was detected in similar defects in an ARE-4 enamel on steel 10.Similar results were noted when the same samples were exposed to a wetsalt environment.

In general, the glass coating 100 has a composition of

(100−x)(0.25Na₂O.0.25B₂O₃.0.5SiO₂).xP₂O₅ in mole percents,

where x is typically between about 5 and 15, more typically betweenabout 5 and 13, and still more typically between about 5 and 9. Theglass formulation typically includes small amounts of one or more of thefollowing oxides, most typically present in amounts of less than about 6mole percent: K₂O, CaO, Al₂O₃, ZrO₂, MnO₂, NiO, and/or CoO.

In operation, one or more (typically a plurality) steel reinforcingmembers 10 are coated with phosphate-rich glass layers 100 foremplacement into a cementitious matrix. The respective steel reinforcingmembers 10 have a first coefficient of thermal expansion and wherein therespective glass layers 100 have a second coefficient of thermalexpansion, typically generally matched to the first coefficient ofthermal expansion. A cementitious matrix 40 is formed around each steelreinforcing member. Once placed into the cementitious environment 40,phosphate is released from the glass coatings 100. The phosphate reactswith the cementitious material 40 to form a hydroxyapatite layer 110 onsteel surfaces 10 without glass coatings 100. The glass coatings 100typically have compositions, in weight percent, of about 40-45% SiO₂,3-5% Al₂O₃, 5-25% B₂O₃, 3-15% K₂O, 5-25% Na₂O, 4-7% CaO, 1-2% ZrO₂, 0-2%NiO, 0-2% CoO, and 5-20% P₂O₅ and each coating 120 typically has acoefficient of thermal expansion between about 12.5 and 13.5 ppm perdegree Celsius.

While the novel technology has been illustrated and described in detailin the drawings and foregoing description, the same is to be consideredas illustrative and not restrictive in character. It is understood thatthe embodiments have been shown and described in the foregoingspecification in satisfaction of the best mode and enablementrequirements. It is understood that one of ordinary skill in the artcould readily make a nigh-infinite number of insubstantial changes andmodifications to the above-described embodiments and that it would beimpractical to attempt to describe all such embodiment variations in thepresent specification. Accordingly, it is understood that all changesand modifications that come within the spirit of the novel technologyare desired to be protected.

We claim:
 1. A corrosion resistant steel reinforcing rod system,comprising: a steel reinforcing rod; and a vitreous shell generallyencapsulating the steel reinforcing rod; wherein the vitreous shell hasa composition selected from the group consisting essentially, in weightpercent, of about 40-45% SiO₂, about 5-25% B₂O₃, about 5-25% Na₂O, andabout 5-20% P₂O₅.
 2. The system of claim 1 wherein the vitreous shellhas a P₂O₅ content of between about 9 and about 18 weight percent. 3.The system of claim 1 wherein the vitreous shell is enveloped in acementitious matrix; wherein there are apertures in the vitreous shellexposing the portions steel rod to the cementitious matrix; whereinphosphate is released from the vitreous shell; and whereinhydroxyapatite is deposited onto exposed portions of the steelreinforcing rod.
 4. A composite structural material comprising incombination: a cementitious matrix; a plurality of steel reinforcingrods positioned in the cementitious matrix; and a plurality of vitreousshells, each respective vitreous shell generally covering a respectivesteel reinforcing rod; wherein each respective steel reinforcing rod hasa coefficient of thermal expansion of between about 14 ppm/° C. andabout 17 ppm/° C.; and wherein each respective vitreous shell has acomposition selected from the group consisting essentially, in weightpercent, of about 40-45% SiO₂, 3-5% Al₂O₃, 5-25% B₂O₃, 3-15% K₂O, 5-25%Na₂O, 4-7% CaO, 1-2% ZrO₂, 0-2% NiO, 0-2% CoO, and 5-20% P₂O₅; andwherein each respective vitreous shell has a coefficient of thermalexpansion between about 12.5 ppm/° C. and about 13.5 ppm/° C.
 5. Thecomposite structural material of claim 4 wherein there are apertures inthe vitreous shell exposing the portions of the steel rods to thecementitious matrix; wherein phosphate is released from the vitreousshell; and wherein hydroxyapatite is deposited onto at least some of theexposed portions of the steel reinforcing rods.
 6. The compositestructural material of claim 4 wherein the bond strength of the steelreinforcing rods in the cementitious matrix increases over time.
 7. Amethod of reinforcing concrete, comprising: coating a plurality of steelreinforcing members with phosphate-rich glass layers for emplacementinto a cementitious matrix, wherein the respective steel reinforcingmembers have a first coefficient of thermal expansion and wherein therespective glass layers have a second coefficient of thermal expansionsubstantially matching the first coefficient of thermal expansion; andforming a cementitious matrix around the plurality of steel reinforcingmembers; releasing phosphate from the glass layers; and forming ahydroxyapatite layer on steel surfaces not coated with glass.
 8. Themethod of claim 7 wherein the glass layers have a composition selectedfrom glasses consisting essentially, in weight percent, of about 40-45%SiO₂, 3-5% Al₂O₃, 5-15% B₂O₃, 3-15% K₂O, 5-20% Na₂O, 4-7% CaO, 1-2%ZrO₂, 0-2% NiO, 0-2% CoO, and 5-20% P₂O₅.
 9. The method of claim 7wherein the glass layers have a coefficient of thermal expansion betweenabout 12.5 and 13.5 ppm per degree Celsius.
 10. The method of claim 7wherein the glass layers contain about 9 to about 18 weight percentP₂O₅.
 11. A steel reinforcing rod system, comprising: a steelreinforcing rod having a coefficient of thermal expansion of betweenabout 14 ppm per degree Celsius and about 17 ppm per degree Celsius; avitreous shell generally encapsulating the reinforcing rod; a pluralityof metal particles distributed throughout the vitreous shell; whereinthe vitreous shell has a composition selected from the group consistingessentially of about 40-45% SiO₂, 3-5% Al₂O₃, 5-25% B₂O₃, 3-15% K₂O,5-25% Na₂O, 4-7% CaO, 1-2% ZrO₂, 0-2% NiO, 0-2% CoO, and 5-20% P₂O₅; andwherein the vitreous shell has a coefficient of thermal expansionbetween about 12.5 ppm per degree Celsius and about 13.5 ppm per degreeCelsius.
 12. The system of claim 11 further comprising a cementitiousmatrix adjacent to and generally surrounding the vitreous shell; whereinthere are apertures in the vitreous shell exposing portions of the steelrods to the cementitious matrix; wherein phosphate is released from thevitreous shell; and wherein hydroxyapatite is deposited onto at leastsome of the portions of the steel reinforcing rods exposed to thecementitious matrix.